1. Field of the Invention
The present invention relates to a fixing device, an image forming apparatus, and a method of controlling a fixing device.
2. Description of the Related Art
An image forming apparatus employing electrophotography includes a fixing device that fixes a toner image transferred onto a recording medium by heating and pressing the toner image. The fixing device includes a fixing member that revolves in the circumferential direction in a heated state and that applies heat to a recording medium that is brought into contact with its outer circumferential face and a pressing member that brings the recording medium into contact with the outer circumferential face of the fixing member in a pressing manner. The fixing device conveys and interposes the recording medium between the fixing member and the pressing member to heat and press the toner image, thereby fixing the toner image positioned on the recording medium.
The fixing devices are categorized into fixing devices of a roller fixing-type and fixing devices of a belt fixing-type. In the roller fixing-type of fixing device, a fixing roller that includes an internal heat source such as a heater is used as the fixing member. In the belt fixing-type of fixing device, an endless fixing belt that revolves in a state heated by an external heat source is used as the fixing member. In either the roller fixing-type or the belt fixing-type of fixing device, a temperature detecting unit is arranged near the heat source or the fixing member, and the temperature of the heat source is controlled in accordance with the temperature detected by the temperature detecting unit.
However, in conventional fixing devices, it is difficult to maintain the temperature of the outer circumferential face of the fixing member to be uniform in the circumferential direction. Therefore, the fixing is sometimes unstable due to the temperature unevenness. Factors causing the temperature unevenness on the outer circumferential face of the fixing member include a difference in distance from a heat source depending on position in the circumferential direction of the outer circumferential face of the fixing member, the occurrence of a local temperature drop due to transfer of heat from the fixing member to a recording medium that is brought into contact with the outer circumferential face of the fixing member, and the like.
When temperature unevenness occurs on the outer circumferential face of the fixing member, the amount of heat applied to toner located on the recording medium changes in accordance with a position in the circumferential direction of the fixing member, and accordingly, a fixing defect is likely to occur. In order to solve such a problem, there is a known method which controls the temperature of a heat source so as to suppress the occurrence of temperature unevenness on the outer circumferential face of the fixing member.
For example, Japanese Patent No. 4170197 discloses a technology in which a fixing member is preheated by operating an internal heat source disposed inside a pressing member in a printing stand-by state, and in returning mode for returning to a printing operation from the printing standby state, a non-heated area at which the temperature of the outer circumferential face of the fixing member is low is moved to a position where heating by a local heat source is performed, the non-heated area is made to stay there for a predetermined time, and power higher than that supplied at the time of a printing operation is input to the local heat source so as to raise the temperature.
However, according to the technology disclosed in Japanese Patent No. 4170197 or many other conventional technologies, the temperature of the heat source is adjusted before performing a printing operation, and the printing operation is started at timing when there is no temperature unevenness. Accordingly, it is difficult to suppress the occurrence of temperature unevenness in the middle of a printing operation. For example, when recording media that are sequentially supplied to the fixing device at the time of consecutive printing operations are repeatedly brought into contact with the same place on the outer circumferential face of the fixing member, a temperature difference between a portion of the fixing member brought into contact with the recording media and a portion of the fixing member that is not brought into contact with the recording media increases, so that temperature unevenness occurs. However, according to the conventional technology in which the temperature of the heat source is adjusted before performing a printing operation, it is difficult to suppress the occurrence of the temperature unevenness in the middle of a printing operation.
In addition, even in a case where temperature unevenness occurring in the middle of a printing operation is tried to be resolved through the adjustment of the temperature of the heat source in the middle of the printing operation, there is a case where the temperature difference between a high-temperature portion of the outer circumferential face of the fixing member and a low-temperature portion thereof may further increase through adjustment of the temperature of the heat source, so that it is difficult to solve the temperature unevenness.